1. Field of the Invention
The present invention relates to the construction of a sheet registering unit disposed upstream of an image transfer stage in an image forming apparatus, e.g., a xerographic machine.
2. Discussion of the Related Art
In image forming apparatuses, such as electronic copying machines and laser based printing machines, a toner image is zerographically formed on an image bearing member, such as a photoreceptor drum. The toner image is transferred to and fixed on a sheet of paper, and output in the form of a copy or print. A sheet registering stage is located upstream of the image transfer stage. The leading edge of the sheet is positioned in a manner that the leading edge is pushed against a gate means in the sheet registering stage. Thereafter, the sheet is fed in synchronism with the toner image on the photoreceptor, so that the toner image is transferred onto the sheet. An example of the sheet registering unit is disclosed in Japanese Patent Unexamined Publication No. Hei. 3-94275. In the disclosed registering unit, registering roller means and gate means are located upstream of the photoreceptor. Preregistering roller means is disposed upstream of the registering roller means, both registering means being separated with a predetermined distance.
The registering unit operates such that the gate means temporarily holds the leading edge of the sheet, and the preregistering roller means additionally feeds the sheet, thereby forming a loop of the sheet. By the force generated in the looped sheet, the sheet leading edge is uniformly pushed against the gate means. Accordingly, when the advancing sheet is obliquely oriented with respect to the advancing direction, the oblique state of the sheet can be removed. In other words, the sheet leading edge is oriented at right angle to the advancing direction in a state that it is nipped by the registering roller means.
The conventional image forming apparatus includes a mechanism functioning such that when a line along which the sheet stops is deviated from the reference line, the registering roller means is laterally moved in accordance with a quantity of the deviation. More specifically, the apparatus laterally moves the registering roller means to make the side of the sheet coincident with the reference line of the sheet advancing path, and drives the registering roller means in synchronism with the toner image on the rotating photoreceptor drum, to move the sheet toward the image transfer stage. As in the conventional image forming apparatus, the image forming apparatus using the linearly-arranged registering unit can form a loop of the sheet by the registering unit and move the registering roller means laterally with respect to the sheet advancing direction by the related mechanism. Accordingly, the apparatus can transport the sheet in a state that the sheet position is corrected with respect to the photoreceptor. The image forming apparatus is free from the problem of the out-of-register of the toner image that is to be transferred onto the sheet.
In some type image forming apparatuses, the registering unit including the sheet transport path cannot be arranged linearly. Accordingly, it must be nonlinearly arranged frequently. The nonlinearly arranged registering unit including the sheet transport path will be described with reference to FIGS. 32 to 40.
In a color copying machine like an image forming apparatus 1 shown in FIG. 32, a sheet of paper is fed out of one of sheet supply trays 10, which are arrayed in a multi-stage fashion, and transported for its register to a sheet registering unit 20, along a sheet transport path 13. A toner image formed on a photoreceptor 3, which takes the form of a drum, is transferred to the supplied sheet in a state that the sheet is held around the photoreceptor 3 located downstream of the registering unit 20.
In the image forming apparatus 1 as the color copying machine, sheet feed means for feeding sheets from the sheet supply tray 10 include a sheet pick-up roller 11 and a sheet feed roller pair 12. A transport roller pair 14 is disposed in the sheet transport path 13. Thus, sheets supplied from the sheet supply tray 10 are transported sheet by sheet toward the registering unit 20. Another sheet feed mechanism including a pick-up roller 16 is provided for feeding sheets supplied from a manual inserter tray 15 into the registering unit. The sheet fed by any of those sheet feed mechanisms is temporarily stopped and then fed toward the transfer drum.
In the registering unit 20, a preregistering roller device 30 is disposed upstream of a curved path 21, and a registering roller device 31 is disposed downstream of the curved path. In operation, to loop the sheet, the preregistering roller device 30 additionally feeds the sheet in a state that the leading edge of the sheet is nipped by the registering roller device 31. The pressure of the looped sheet functions to remove the oblique state of the sheet. A pressure roller 5 presses the sheet emanating from the registering roller device 31 against the surface of the transfer drum 2, and in this state the sheet is electrostatically attracted to the surface of the transfer drum 2. Every time a color toner image is formed on the surface of the transfer drum 2, the color toner image is transferred onto the sheet. The sheet holding member of the transfer drum 2 is made of a plastic film. The transfer drum 2 is further provided with a means for causing the plastic film to electrostatically hold the sheet.
The image forming apparatus shown in FIG. 32, the registering roller device 31 is the combination of a drive roller 32 and a pinch roller 42, as shown in FIG. 33. The registering roller device 31 moves forward the sheet in a state that it nips the sheet. As shown, the drive roller 32 is fastened to a predetermined location of a drive shaft 34. The pinch roller 42 is mounted around a shaft 44.
In the sheet transporting system shown in FIG. 32, various types of roller means are disposed along the sheet transport path 13. The sheet is transported through the pick-up roller 11, the transport roller pair 14, the preregistering roller device 30, and the registering roller device 31, and finally is pressed against the transfer drum 2 by means of the pressure roller 5.
In the sheet transport path (FIG. 34), a loop of the sheet is formed between both the roller devices 30 and 31 in the registering unit 20, as shown in FIG. 35. A film 24 is disposed at the corner of the inner side of the curved path 21. The film 24 pushes back the loop of the sheet, so that the leading edge of the sheet is pushed against the registering roller device 31. Thus, the film 24 functions to assist the loop of the sheet in regulating the orientation of the leading edge of the sheet. Through the regulation of the leading edge orientation, as shown in FIG. 32, the sheet fed out of the registering unit 20 is pressed against the transfer drum 2 and is held by the drum, and the sheet is set on the surface of the transfer drum 2 in good conditions. To be more specific, when a sheet of paper P obliquely enters the registering unit 20 as shown in FIG. 36, the sheet advances between the roller devices 30 and 31 in the oblique state. The sheet further advances and its leading edge reaches the preregistering roller device 30. At this time, the sheet is temporarily stopped there while the sheet is additionally fed by the preregistering roller device 30. As the result of the additional feeding operation, the sheet is looped. A reaction force of the looped sheet forcibly aligns the leading edge of the sheet with the nip extension of the registering roller device 31, which is at a right angle to the leading edge of the sheet. In this way, even the obliquely advancing sheet can be normally set onto the drum surface.
When the sheet P, which is in a state as shown in FIG. 37, is moved toward the transfer drum 2, the sheet is pressed against the transfer drum 2 while undergoing a twisted force, and is electrostatically set on the drum surface. When the sheet is set on the drum surface, a reaction force in the twisting direction deforms the film member of the transfer drum 2 in its width direction, as shown in FIG. 39. The side P1 of the sheet supported by the drum is slightly displaced from the reference line a-1 along which the sheet should be held around the drum. As shown in FIG. 39, when the trailing edge of the sheet being held in the displaced state has passed the registering roller device 31, the reaction force of the twisting direction disappears. After the pressure of the sheet is removed, the reaction of the film causes the sheet to displace from the original position where the sheet has first been set, in the width direction. As a result, the sheet is returned to the position of the reference line a-1.
In the color copying machine as described above, when the leading edge of the sheet is held by the transfer drum 2 and the operation for setting the sheet on the drum continues, the transfer operation of a first color toner image is started. Before a second color toner image is transferred, the sheet is moved widewise by the film reaction force. As a result, the line Pa of the first color toner image is not aligned with the line Pb of the second color toner image with respect to the sheet advancing direction (see FIG. 40). In other words, a discrepancy in color is caused. A third color toner image to be transferred subsequent to the second color toner image, is also transferred along the same line as that of the second color toner image.
As described above, the sheet transporting system suffers from the discrepancy-in-color problem, or deterioration of the copy quality. The lateral displacement of the sheet during the transferring process of the first color toner image will possibly cause line displacement and image displacement in the first color toner image, and an improper transfer of the image.
One of the feasible ways to solve the above problem is to turn the transfer drum idle after the sheet is set on the drum, thereby removing the distortion of the film, and then to transfer the color toner image. The idle turning of the drum with the sheet held thereon leads to diminishing of the number of copies made per unit time.